The present invention relates generally to fuel delivery systems for gas turbine engines and more specifically to an inverse fuel model and method for implementing liquid fuel flow control in a gas turbine to achieve a nearly bump-less driven watts (dwatt) power output during fuel mode transitions between passive and active modes of operation of a three-way check valve which delivers liquid fuel to the turbine combustor.
A gas turbine engine includes a compressor, combustor and turbine. Compressed air is delivered by the compressor to the combustor in which fuel is mixed with the air and combusted. Hot combustion gases turn the turbine that drives the compressor and generates work from the gas turbine engine. The combustor is formed of combustion cans typically arranged in an annular array between the compressor and turbine. Fuel to the combustor flows through pipes and valves that meter the fuel to the combustion cans. The valves are used to control fuel flow and to ensure that fuel flows equally to each of the combustion cans.
Industrial gas turbines are often capable of alternatively running on liquid and gaseous fuels, e.g., natural gas. These gas turbines have fuel supply systems for both liquid and gas fuels. The gas turbines generally do not burn both gas and liquid fuels at the same time. Rather, when the gas turbine burns liquid fuel, the gas fuel supply is turned off. Similarly, when the gas turbine burns gaseous fuel, the liquid fuel supply is turned off. Fuel transfers occur during the operation of the gas turbine as the fuel supply is switched from liquid fuel to gaseous fuel, and vice versa.
Gas turbines that burn both liquid and gaseous fuel require a liquid fuel purge system to clear the fuel nozzles in the combustors of liquid fuel. The liquid fuel supply system is generally turned off when a gas turbine operates on gaseous fuel. When the liquid fuel system is turned off, the purge system operates to flush out any remaining liquid fuel from the nozzles of the combustor and provide continuous cooling airflow to the nozzles.
FIG. 1 is a simplified schematic diagram of an exemplary gas turbine having liquid and gas fuel systems. FIG. 1 shows schematically a gas turbine power generation system 100 having liquid fuel system 102 and a liquid fuel purge system 104. The gas turbine is also capable of running on a gas, such as natural gas, and includes a gaseous fuel system 106. Other major components of the gas turbine include a main compressor 108, a combustor 110, a turbine 112 and a system controller 114. The power output of the gas turbine 112 is a rotating turbine shaft 116, which may be coupled to a generator 130 that produces electric power.
In the exemplary industrial gas turbine shown, the combustor may be an annular array of combustion chambers, i.e., combustion cans 118, each of which has a liquid fuel nozzle 120 and a gas fuel nozzle 122. The combustor may alternatively be an annular chamber. Combustion is initiated within the combustion cans at points slightly downstream of the nozzles. Air from the compressor 108 flows around and through the combustion cans 118 to provide oxygen for combustion. Moreover, water injection nozzles 124 are arranged within the combustor 110 to add excess mass flow to the hot combustion gases and to cool the combustion cans 118. The air for the liquid fuel system purge may be provided from the compressor 108, boosted by a purge air compressor (not shown) and controlled by other elements of the system (not shown). When the gas turbine power generation system 100 operates on natural gas (or other gaseous fuel), the liquid fuel purge system 104 blows compressed air into the combustion cans 118 through the liquid fuel nozzles 120 of the liquid fuel 102 system to purge liquid fuel and provide a flow of continuous cooling air to the liquid fuel nozzles 120.
FIG. 2 is a simplified diagram of a gas turbine engine with an existing liquid fuel system. Liquid fuel is provided to the liquid fuel system 200 from a liquid fuel source 205. The liquid fuel system 200 includes a flow path to a flow divider 230 through a low pressure filter 210, a fuel pump 215, a bypass control valve 220, and a stop valve 225. Pressure relief valve 235, bypass control valve 220 and stop valve 225 serve to recirculate liquid fuel to the upstream side of the low pressure filter 210 and regulate flow to flow divider 230 and fuel delivery to three-way check valve(s) 245. The flow divider 230 divides liquid fuel flow into a plurality of liquid fuel flow paths leading to one or more three-way check valve(s) 245 which feed fuel to individual combustion cans 270 of the turbine.
The turbine system controller 114 provides control signals to the fuel pump and each of the various valves to regulate and control fuel flow that is provided to the combustors in response to a fuel reference demand for a given power output. Conventionally, the controller 114 may include, among other things, an output control signal for initiating a predetermined liquid fuel prefill flow rate through the liquid fuel system, an output control signal for controlling transitions of a fuel delivery three-way valve 245 between purge air delivery and liquid fuel operation, and an output control signal for controlling a fuel bypass control valve 220 for regulating fuel flow to a fuel flow divider 230 and a turbine combustor can. The controller 114 may also accept input signals from various turbine system sensors and incorporate a hardware processor for implementing an algorithm to generate appropriate control signals based on sensor inputs and measured system parameters such as a Driven Megawatts power output.
Each liquid fuel flow path downstream of the flow divider includes a combustor fuel delivery three-way check (endcover) valve 245 (three-way valve) and a distribution valve 260 before entering a combustor combustion can 270. Three-way valve 245 permits flow to the combustion can nozzles from the liquid fuel flow path (described above) or air flow from a liquid fuel purge air system 280. Three-way valve 245 is designed to selectably allow fuel flow to the combustor nozzles 120 from a liquid fuel supply system while preventing backflow of fuel into the liquid fuel purge air system or to allow purge air to the combustor nozzles 120 while preventing backflow of purge air into the liquid fuel system upstream of the three-way valve. By preventing purge air from entering the liquid fuel system, the air-fuel interfaces with the fuel supply are minimized.
When gas (gaseous) fuel is supplying the turbine, the three-way valve 245 is positioned to block liquid fuel flow and allow purge air to pass for cooling the fuel nozzles in the combustor. This purge must be shut off when liquid fuel is turned on.
The three-way valve 245 has passive and active operational modes. During the active mode, three-way valve 245 is controlled by external forces, such as a “Pilot” (instrument) air pressure applied by the turbine system controller 114. In passive mode, the three-way valve is controlled by the pressure of the liquid fuel. The passive mode is used to switch the three-way valve between purge air flow and purge liquid fuel flow. The active mode is applied to hold the three-way valve in a liquid fuel ON flow setting during high fuel-flow conditions. The active mode is not used to switch the three-way valve from fuel flow to purge air, or vice versa. Three-way valve 245 is biased to purge air flow, if there is insufficient fuel pressure present to operate the valve. The three-way valve 245 (operating in the passive mode) automatically switches to pass fuel to the combustor fuel nozzles when the fuel pressure increases. The increase in fuel pressure itself is the actuating force that switches the three-way valve from applying purge air to applying liquid fuel flow to the combustor.